Particle Sinking Velocity Research Articles

Seasonal variations of sinking velocities in Austral diatom blooms: Lessons learned from COMICS

The sinking velocity (SV) of organic particles is a critical driver of carbon transport to the deep sea. Accurate determination of marine particle SV and their influencing factors is therefore a key to better understanding of biological carbon storage in the ocean. We used two different approaches to estimate average SVs of particles during a Southern Ocean spring bloom (North of South Georgia): optical backscatter sensors on gliders (“large”, >50 μm diameter), and radioactive pairs (234Th–238U and 210Po-210Pb). Our results were complemented with time-of flight estimations of bulk SVs from deep sediment traps deployed at 1950 m.Bulk SVs increased consistently with depth from 15 ± 1 m d−1 at 10 m to 50 ± 10 m d−1 at the depth of export (Zp = 95 m) and from 96 ± 35 m d−1 at 150 m to 119 ± m d−1 at 450 m. Only the fastest particles, mainly comprised by faecal pellets (FPs) and diatom aggregates, survived remineralization and dominated carbon fluxes at deep depth.The SV variability at the base of the Euphotic Zone was studied in relation to the stage of the bloom by analysing three different moments of the spring diatom bloom in the region during the years 2012, 2013 and 2017. The export efficiency (ExpEff), defined as the ratio POC flux exported below the Euphotic Zone to the satellite derived surface NPP, was also evaluated. It was found from the temporal series that ExpEff and SV vary throughout the diatom bloom as the community structure progresses. A good correlation between both variables was observed (ExpEff = (0.023 ± 0.006) SV, r = 0.82, p = 0.04). Showing that the variability in how efficiently the carbon flux is exported out of the Euphotic Zone can be explained by the SV at which the particles sink. Further investigations are required to analyse if this is a specific model of the functioning of the BCP during the diatom bloom in North South Georgia or if it can be extrapolated to other scenarios.

The importance of transparent exopolymer particles over ballast in determining both sinking and suspension of small particles during late summer in the Northeast Pacific Ocean

Gravitational sinking of particles is a key pathway for the transport of particulate organic carbon (POC) to the deep ocean. Particle size and composition influence particle sinking velocity and thus play a critical role in controlling particle flux. Canonically, sinking particles that reach the mesopelagic are expected to be either large or ballasted by minerals. However, the presence of transparent exopolymer particles (TEP), which are positively buoyant, may also influence particle sinking velocity. We investigated the relationship between particle composition and sinking velocity during the Export Processes in the Ocean from RemoTe Sensing (EXPORTS) campaign in the Northeast Pacific Ocean using Marine Snow Catchers. Suspended and sinking particles were sized using FlowCam for particle imaging, and their biogeochemical composition was assessed by measuring the concentration of particulate organic carbon (POC) and nitrogen, particulate inorganic carbon, biogenic and lithogenic silica, and TEP. Sinking fluxes were also calculated. Overall, both suspended and sinking particles were small (<51 μm, diameter) in this late summer, oligotrophic system. Contrary to expectation, the ratio of ballast minerals to POC was higher for suspended particles than sinking particles. Further, suspended particles showed TEP-to-POC ratios three times higher than sinking particles. These ratios suggest that TEP content and not ballast dictated whether particles in this system would sink (low TEP) or remain suspended (high TEP). Fluxes of POC averaged 4.3 ± 2.5 mmol C m−2 d−1 at 50 m (n = 9) and decreased to 3.1 ± 1.1 mmol C m−2 d−1 at 300–500 m (n = 6). These flux estimates were slightly higher than fluxes measured during EXPORTS with drifting sediment traps and Thorium-234. A comparison between these approaches illustrates that small sinking particles were an important component of the POC flux in the mesopelagic of this late summer oligotrophic system.

Drivers of particle sinking velocities in the Peruvian upwelling system

Abstract. As one of Earth's most productive marine ecosystems, the Peruvian upwelling system transports large amounts of biogenic matter from the surface to the deep ocean. Whilst particle sinking velocity is a key factor controlling the biological pump, thereby affecting carbon sequestration and O2-depletion, it has not yet been measured in this system. During a 50 d mesocosm experiment in the surface waters off the coast of Peru, we assessed particle sinking velocities and their biogeochemical and physical drivers. We further characterized the general properties of exported particles under different phytoplankton communities and nutritional states. Average sinking velocities varied between size classes and ranged from 12.8 ± 0.7 m d−1 (particles 40–100 µm) to 19.4 ± 0.7 m d−1 (particles 100–250 µm) and 34.2 ± 1.5 m d−1 (particles 250–1000 µm) (± 95 % CI). Despite a distinct plankton succession from diatoms to dinoflagellates with concomitant 5-fold drop in opal ballasting, substantial changes in sinking velocity were not observed. This illustrates the complexity of counteracting factors driving the settling behaviour of marine particles. In contrast, we found higher sinking velocities with increasing particle size and roundness and decreasing porosity. Size had by far the strongest influence among these physical particle properties, despite a high amount of unexplained variability. Our study provides a detailed analysis of the drivers of particle sinking velocity in the Peruvian upwelling system, which allows modellers to optimize local particle flux parameterization. This will help to better project oxygen concentrations and carbon sequestration in a region that is subject to substantial climate-driven changes.

Deflagration characteristics of freely propagating flames in magnesium hydride dust clouds

The flame propagation processes of MgH2 dust clouds with four different particle sizes were recorded by a high-speed camera. The dynamic flame temperature distributions of MgH2 dust clouds were reconstructed by the two-color pyrometer technique, and the chemical composition of solid combustion residues were analyzed. The experimental results showed that the average flame propagation velocities of 23 μm, 40 μm, 60 μm and 103 μm MgH2 dust clouds in the stable propagation stage were 3.7 m/s, 2.8 m/s, 2.1 m/s and 0.9 m/s, respectively. The dust clouds with smaller particle sizes had faster flame propagation velocity and stronger oscillation intensity, and their flame temperature distributions were more even and the temperature gradients were smaller. The flame structures of MgH2 dust clouds were significantly affected by the particle sinking velocity, and the combustion processes were accompanied by micro-explosion of particles. The falling velocities of 23 μm and 40 μm MgH2 particles were 2.24 cm/s and 6.71 cm/s, respectively. While the falling velocities of 60 μm and 103 μm MgH2 particles were as high as 15.07 cm/s and 44.42 cm/s, respectively, leading to a more rapid downward development and irregular shape of the flame. Furthermore, the dehydrogenation reaction had a significant effect on the combustion performance of MgH2 dust. The combustion of H2 enhanced the ignition and combustion characteristics of MgH2 dust, resulting in a much higher explosion power than the pure Mg dust. The micro-structure characteristics and combustion residues composition analysis of MgH2 dust indicated that the combustion control mechanism of MgH2 dust flame was mainly the heterogeneous reaction, which was affected by the dehydrogenation reaction.

Effects of riverine nutrient inputs on the sinking fluxes of microbial particles in the St. Lawrence Estuary

Discharges of nitrogenous nutrients associated with agriculture and wastewater treatment promote eutrophication in coastal waters around the world. Since the Lower St. Lawrence Estuary shows early signs of eutrophication, with the presence of hypoxic and acidified deep waters, a mesocosm experiment was conducted to determine the impact of variations of nitrogenous nutrients from riverine inputs to this estuarine environment during the spring–summer phytoplankton growth season. Four treatments were applied to recreate conditions mimicking 1) upwelled waters at the head of the Lower Estuary (“control”), 2) a mix of upwelled and riverine water (”+N”), 3) the same mix of upwelled and riverine waters with a greater proportion of urea in the total nitrogenous pool (”+Urea”), and 4) waters with a 50% decrease in nitrogenous riverine inputs (”–N”). The composition and vertical fluxes of microbial communities were characterized under these four scenarios. Nitrogenous nutrients were consumed within three days in all treatments. A higher concentration of nitrogenous nutrients led to increases in phytoplankton carbon biomass. The centric diatom Skeletonema costatum dominated in all treatments. The microbial community primarily consumed nitrate in all treatments, but equally preferred nitrate and urea in the +Urea treatment. Mean particle sinking velocities were not statistically different between the two most nitrogen-enriched treatments and the control, but the higher dissolved silicon:nitrate uptake ratio in the –N treatment could have favored silicification of the diatoms, leading to increased sinking speeds. Indeed, the total biogenic carbon sinking flux in the –N was on average 51% higher than in the other treatments (∼391 mg C m−2 d−1 vs ∼253–262 mg C m−2 d−1). These results indicate that efficient control measures concerning the St. Lawrence watershed should consider concentrations and ratios of nutrients that potentially promote phytoplankton growth in the Lower Estuary and consequently increase the amount of carbon exported to and decomposed in hypoxic and acidified deep layers.

Comparing two ocean biogeochemical models of Chesapeake Bay with and without the sulfur cycle instead highlights the importance of particle sinking, burial, organic matter, nitrification and light attenuation

A number of models have been developed to simulate hypoxia in the Chesapeake Bay, but these models vary in complexity and in which processes they represent. In this study we implement a previously published biogeochemical code (BioRedoxCNPS) developed for open-ocean waters that includes “cryptic” microbial sulfur cycling within the ChesROMS physical model of the Chesapeake Bay. Sulfur cycling can increase rates of denitrification and anammox in anoxic waters, but the net impacts of such changes on oxygen, nitrate and ammonium are not understood. We compare the results to a physically identical simulation with an estuarine biogeochemical cycling code previously implemented and calibrated in the Bay (ECB). The ECB code neglects sulfur cycling but includes burial of particulate organic matter (POM) and cycling of dissolved organic matter (DOM) and uses different values for many parameters governing phytoplankton growth and particle dynamics. Although the BioRedoxCNPS model produces a better simulation of oxygen and nitrate at a key test site this turns out not to be due to the inclusion of sulfur cycling. Instead, large differences in modeled oxygen and ammonium are largely due to whether or not the biogeochemical codes include cycling of DOM and sedimentary burial of POM. Changes in light attenuation produce large changes in nitrate. Changes in parameters used in both biogeochemical codes (in particular particle sinking velocities) tended to compensate the other differences in model construction. The quantitative impacts of these choices for simulating Chesapeake Bay have not previously been documented in the peer-reviewed literature. Predictions of hydrogen sulfide from our merged model were very sensitive to the choice of parameters and light attenuation. This suggests that observations of hydrogen sulfide could help to constrain these processes in future models.

In Situ Particle Measurements Deemphasize the Role of Size in Governing the Sinking Velocity of Marine Particles.

Sinking particles are important in delivering carbon to the deep ocean where it may be stored out of contact with the atmosphere. Whilst particle sinking velocities are known to be influenced by a multitude of factors, size-based parameterizations remain common in biogeochemical models and in the methods used to determine particulate fluxes from autonomous platforms. Here we carried out an extensive literature review (62 data sets) into the size-sinking velocity relationship, and find the relationship is much weaker for studies examining particles in situ (median R 2=0.09) compared with ex situ studies (median R 2=0.35). This discrepancy may be because particles examined in the laboratory have more uniform properties than those studied in situ. Our review highlights the shortcomings of using a simple relationship between size and sinking velocity to calculate sinking particulate fluxes in the ocean; considering additional particle characteristics will enable more accurate calculations of particulate fluxes.

Slow-sinking particulate organic carbon and its attenuation in the mesopelagic water of the South China Sea

Coastal acidification has been widely investigated in terms of its rationale and ecological effects in the last decade. However, the driving mechanism for acidification in open seawater, especially in mesopelagic water, is still poorly understood. Here, the sinking velocity and flux attenuation of particulate organic carbon (POC) were examined based upon the radioactive 210Po-210Pb tracer to reveal the remineralization of POC in the mesopelagic zone in the northeastern South China Sea (SCS). Overall, the profiles of 210Po followed those of 210Pb, lending support to the particle sinking controlled top-down deficits of 210Po. Using an inverse model, the sinking velocity of particles, for the first time in the SCS, was estimated to vary from 3 to 34 m d-1 with the mean value of 15 ± 9 m d-1, indicating that the slow sinking particles largely contribute to the POC flux in the SCS. Beneath the euphotic zone, a consistent descending of the sinking speed implied continuous remineralization of sinking POC in the twilight zone. A preliminary estimate revealed that 1.9-5.4 mmol-C m-2 d-1 remineralized back to carbon dioxide within 100-500 m, representing about 70% of the exported autochthonous POC from the euphotic zone. In 100-1000 m, 2.4-6.6 mmol-C m-2 d-1 (i.e., 84%) remineralized. Thus, the upper twilight zone (i.e., 100-500 m) is the dominant layer of POC remineralization, and POC-induced acidification could be unneglectable there. These results provided insights into the POC-induced acidification mechanism in the mesopelagic water, especially in the upper mesopelagic layer.

Sinking velocity of small particles in the Black Sea: Vertical distribution and seasonal variability from continuous Bio-Argo measurements of backscattering

High-resolution Bio-Argo measurements of backscattering ( bbp ) were used to study the vertical distribution and time variability of the sinking of small, highly reflective particles in the Black Sea in the 0–200 m layer. The evolution of particles is divided into three stages: formation, sinking, and removal. The formation of small, highly reflective particles is mainly related to early summer and early winter coccolithophore blooms and the formation of suspended oxidized manganese in the suboxic zone. The removal rate of the particles was estimated from the vertical distribution of bbp in relation to its values at the layers of particle formation. Further, contour analysis of the time-depth diagram of bbp anomalies was used to automatically estimate the vertical distribution and seasonal and interannual variability of sinking velocity ( w ). The average value of w is ~0.8 m/day, which agrees with laboratory estimates of coccolith sinking ( Fritz and Balch, 1996 ). Physical processes significantly affect w : mixed layer deepening in late autumn-early winter increase w up to 1–1.5 m/day; warming and a decrease in density of upper layers promote coccolithophores deepening in summer ( w = 0.9–1.1 m/day). Minimum sinking velocity is observed in the warm period of a year in the strongly stratified layers of the main pycnocline at 40–80 m. On interannual time scales, we observed an increase in w during the strongest coccolithophore blooms, which is probably related to the enhanced particle aggregation. Sinking particles after winter coccolithophore blooms are traced up to the suboxic zone, where it probably affects the settling of manganese-containing particles and related biochemical processes. • Temporal and vertical variability of sinking velocity (w) is estimated using Bio-Argo bbp data. • Mixed layer deepening, stratification, and warming affect variations in w. • The estimated average sinking velocity of coccoliths is ~0.8 m/day. • Winter particles formed in the upper layers affect particles dynamics in the suboxic zone. • On interannual time scales w increases during strongest coccolithophore blooms.

Impact of mesoscale eddies on the source funnel of sediment trap measurements in the South China Sea

The mooring tethered time-series sediment traps (TS-traps) collect sinking particles in the ocean, enabling the estimation of biological pump (BP) efficiency based on the assumption of vertical settling. However, the advection of sea water can disperse particles over long distance during the sinking process, introducing uncertainties into the estimates of BP efficiency. In the South China Sea, mesoscale eddies generate significant lateral transport above the mesopelagic zone, which has a large impact on the collecting area and source funnel of TS-traps. We studied the role of eddies in the lateral transport of sinking particles using a Lagrangian Particle Tracking Model (LPTM) which was forced by three-dimensional currents of eddy composites derived from the global Hybrid Coordinate Ocean Model (HYCOM). The eddy reanalysis using the HYCOM outputs was validated using occurrence, genesis and dissipation statistics of eddies. Backward particle tracking was performed at three stations located in the northern, central and southwestern SCS, respectively. The results indicate that the vertical sinking assumption is invalid when the particle sinking velocity is relatively low. Furthermore, the fraction of surface sources from the edge of the eddies depends on both the sinking velocity and seasonal variability of the background current. The eddy current controls the movement of particles within the eddy, and many of the particles with a sinking velocity <80 m d−1 can be transported from the edge to the eddy center. The source funnel within the mesoscale eddy is tilted vertically, leading to an asymmetrical distribution of the particle source around the trap location. Finally, for slow-sinking particles, trap-collected particles could originate from the subsurface located tens to hundreds of kilometers away from the trap location. Overall, our results highlight the role of mesoscale eddies in interpreting trap measurements.

Size does not matter after all: No evidence for a size-sinking relationship for marine snow

The biological carbon pump is a critical component for uptake of carbon by the oceans. Most of this is mediated by gravitational sinking of particles and it is generally assumed that there is a positive relationship between sinking velocities and sizes of particles. Due to the difficulties inherent in measuring the sinking velocity of untouched and undamaged particles, the majority of studies have been based on artificial solid particles or laboratory generated marine snow formed from homogenous material, e.g. phytoplankton cultures. Here, we present results from a newly developed optical method that measures sizes and sinking velocities of undisturbed in situ aggregates in the mesopelagic zone. The measurements were done at depths between 90 and 530 m throughout day and night. In total we measured 55 image sequences of aggregate sizes and sinking velocities, resulting in measurements of 1060 individual aggregates. Only 10 sequences showed significant correlations between aggregate size and sinking velocity. Furthermore, only 8 of the 10 significant correlations between size and settling were positive. Despite similar optical appearance, similar sized aggregates had different sinking velocities, suggesting that aggregates formed in situ are heterogeneous with different compositions, structure and densities. This complicates estimates of sinking velocity from aggregate size only and one has to be careful when estimating downward particle flux based on in situ size-distribution and abundance of aggregates and theoretical relationships between size and sinking velocity. Hence, this study forces the conclusion that estimates of downward particle flux which use particle size distributions must also include information about aggregate composition, compactness and density from in situ optics or direct particle and aggregate sampling.

New guidelines for the application of Stokes' models to the sinking velocity of marine aggregates

AbstractNumerical simulations of ocean biogeochemical cycles need to adequately represent particle sinking velocities (SV). For decades, Stokes' Law estimating particle SV from density and size has been widely used. But while Stokes' Law holds for small, smooth, and rigid spheres settling at low Reynolds number, it fails when applied to marine aggregates complex in shape, structure, and composition. Minerals and zooplankton can alter phytoplankton aggregates in ways that change their SV, potentially improving the applicability of Stokes' models. Using rolling cylinders, we experimentally produced diatom aggregates in the presence and absence of minerals and/or microzooplankton. Minerals and to a lesser extent microzooplankton decreased aggregate size and roughness and increased their sphericity and compactness. Stokes' Law parameterized with a fractal porosity modeled adequately size‐SV relationships for mineral‐loaded aggregates. Phytoplankton‐only aggregates and those exposed to microzooplankton followed the general Navier‐Stokes drag equation suggesting an indiscernible effect of microzooplankton and a drag coefficient too complex to be calculated with a Stokes' assumption. We compared our results with a larger data set of ballasted and nonballasted marine aggregates. This confirmed that the size‐SV relationships for ballasted aggregates can be simulated by Stokes' models with an adequate fractal porosity parameterization. Given the importance of mineral ballasting in the ocean, our findings could ease biogeochemical model parameterization for a significant pool of particles in the ocean and especially in the mesopelagic zone where the particulate organic matter : mineral ratio decreases. Our results also reinforce the importance of accounting for porosity as a decisive predictor of marine aggregate SV.

A microscopic simulation of particle flux in ocean waters: Application to radioactive pair disequilibrium

The biological carbon pump, a fundamental component of the global carbon cycle, is a mechanism comprised of biological processes that transfer large amounts of organic carbon from the upper ocean to the ocean’s interior. It sequesters CO2 for weeks to hundreds or even millions of years through the sinking of a complex mix of biogeo-chemical material. As particles sink the flux of particulate organic carbon attenuates and only a fraction of the flux reaches the ocean’s depth. Quantifying the attenuation of sinking particulate carbon flux is key to assess the amount of carbon sequestered in the twilight zone. Current knowledge of the particle flux attenuation relies on a reduced number of direct measurements, usually fitted to a power law, that tries to describe the result of a large variety of biological, physical and chemical processes involved.We present a stochastic simulation for particle production, sinking and degradation in the oceans that includes most of the essential ingredients to ultimately describe the sinking carbon flux in the ocean and where all the complex biogeochemical behavior is captured by macroscopic parameters. The algorithm proposed interprets these phenomena probabilistically and is an ideal framework to describe the observed patterns of particle flux attenuation in depth. Particles are randomly produced and degraded with depth-dependent functions, which reproduces the variability of these processes in nature. Hence, the simulation proposed provides a suitable approach to analyse sinking carbon when different particle sizes or speeds are used. Furthermore, production and degradation phenomena are time-dependent, this way their seasonal evolution can be additionally explored and the simulation results in an ideal test-bed for the assessment of both steady and non-steady state situations. In addition, the theoretical formulation proposed provides a simple frame to include particle-metals interactions through adsorption and desorption processes.In this paper we relate typical parametrizations of the flux attenuation curve to their corresponding degradation rate and propose an alternative parametrization to model the flux degradation function which is negligible at very large depths, therefore producing a constant residual flux, and remains finite in the euphotic zone. This degradation function is implemented in the simulation and applied to estimate the scavenging of the naturally occurring radioisotopes 234Th and 210Po. We use the results to estimate effective adsorption and desorption rate constants for both isotopes. We found that the degradation rate, rather than the half-life, controls the depth at which the parent and daughter reach equilibrium. Thus, this depth is ruled by a combination of the degradation rate and the particle sinking velocity.

In situ camera observations reveal major role of zooplankton in modulating marine snow formation during an upwelling-induced plankton bloom

Particle aggregation and the consequent formation of marine snow alter important properties of biogenic particles (size, sinking rate, degradability), thus playing a key role in controlling the vertical flux of organic matter to the deep ocean. However, there are still large uncertainties about rates and mechanisms of particle aggregation, as well as the role of plankton community structure in modifying biomass transfer from small particles to large fast-sinking aggregates.Here we present data from a high-resolution underwater camera system that we used to observe particle size distributions and formation of marine snow (aggregates >0.5 mm) over the course of a 9-week in situ mesocosm experiment in the Eastern Subtropical North Atlantic. After an oligotrophic phase of almost 4 weeks, addition of nutrient-rich deep water (650 m) initiated the development of a pronounced diatom bloom and the subsequent formation of large marine snow aggregates in all 8 mesocosms. We observed a substantial time lag between the peaks of chlorophyll a and marine snow biovolume of 9–12 days, which is much longer than previously reported and indicates a marked temporal decoupling of phytoplankton growth and marine snow formation during our study. Despite this time lag, our observations revealed substantial transfer of biomass from small particle sizes (single phytoplankton cells and chains) to marine snow aggregates of up to 2.5 mm diameter (ESD), with most of the biovolume being contained in the 0.5–1 mm size range. Notably, the abundance and community composition of mesozooplankton had a substantial influence on the temporal development of particle size spectra and formation of marine snow aggregates: While higher copepod abundances were related to reduced aggregate formation and biomass transfer towards larger particle sizes, the presence of appendicularia and doliolids enhanced formation of large marine snow.Furthermore, we combined in situ particle size distributions with measurements of particle sinking velocity to compute instantaneous (potential) vertical mass flux. However, somewhat surprisingly, we did not find a coherent relationship between our computed flux and measured vertical mass flux (collected by sediment traps in 15 m depth). Although the onset of measured vertical flux roughly coincided with the emergence of marine snow, we found substantial variability in mass flux among mesocosms that was not related to marine snow numbers, and was instead presumably driven by zooplankton-mediated alteration of sinking biomass and export of small particles (fecal pellets).Altogether, our findings highlight the role of zooplankton community composition and feeding interactions on particle size spectra and formation of marine snow aggregates, with important implications for our understanding of particle aggregation and vertical flux of organic matter in the ocean.

Competitive fitness of a predominant pelagic calcifier impaired by ocean acidification

Coccolithophores—single-celled calcifying phytoplankton—are an important group of marine primary producers and the dominant builders of calcium carbonate globally. Coccolithophores form extensive blooms and increase the density and sinking speed of organic matter via calcium carbonate ballasting. Thereby, they play a key role in the marine carbon cycle. Coccolithophore physiological responses to experimental ocean acidification have ranged from moderate stimulation to substantial decline in growth and calcification rates, combined with enhanced malformation of their calcite platelets. Here we report on a mesocosm experiment conducted in a Norwegian fjord in which we exposed a natural plankton community to a wide range of CO2-induced ocean acidification, to test whether these physiological responses affect the ecological success of coccolithophore populations. Under high-CO2 treatments, Emiliania huxleyi, the most abundant and productive coccolithophore species, declined in population size during the pre-bloom period and lost the ability to form blooms. As a result, particle sinking velocities declined by up to 30% and sedimented organic matter was reduced by up to 25% relative to controls. There were also strong reductions in seawater concentrations of the climate-active compound dimethylsulfide in CO2-enriched mesocosms. We conclude that ocean acidification can lower calcifying phytoplankton productivity, potentially creating a positive feedback to the climate system.

The wineglass effect shapes particle export to the deep ocean in mesoscale eddies

AbstractMesoscale eddies in the ocean strongly impact the distribution of planktonic particles, mediating carbon fluxes over ~1/3 of the world ocean. However, mechanisms controlling particle transport through eddies are complex and challenging to measure in situ. Here we show the subsurface distribution of eddy particles funneled into a wineglass shape down to 1000 m, leading to a sevenfold increase of vertical carbon flux in the eddy center versus the eddy flanks, the “wineglass effect”. We show that the slope of the wineglass (R) is the ratio of particle sinking velocity to the radially inward velocity, such that R represents a tool to predict radial particle movement (here 0.05 m s−1). A simple model of eddy spindown predicts such an ageostrophic flow concentrating particles in the eddy center. We explore how size‐specific particle flux toward the eddy center impacts eddies' biogeochemistry and export fluxes.

Geographical, seasonal, and depth variation in sinking particle speeds in the North Atlantic

AbstractParticle sinking velocity is considered to be a controlling factor for carbon transport to the deep sea and thus carbon sequestration in the oceans. The velocities of the material exported to depth are considered to be high in high‐latitude productive systems and low in oligotrophic distributions. We use a recently developed method based on the measurement of the radioactive pair 210Po‐210Pb to calculate particle sinking velocities in the temperate and oligotrophic North Atlantic during different bloom stages. Our estimates of average sinking velocities (ASVs) show that slowly sinking particles (&lt;100 m d−1) contribute significantly to carbon flux at all the locations except in the temperate regions during the bloom. ASVs appear to vary strongly with season, which we propose is caused by changes in the epipelagic community structure. Our results are the first field data to confirm the long‐standing theory that particle sinking velocities increase with depth, with increases of up to 90% between 50 and 150 m depth.

230Th and 234Th as coupled tracers of particle cycling in the ocean: A maximum likelihood approach

We applied maximum likelihood estimation to measurements of Th isotopes (234,230Th) in Mediterranean Sea sediment traps that separated particles according to settling velocity. This study contains two unique aspects. First, it relies on settling velocities that were measured using sediment traps, rather than on measured particle sizes and an assumed relationship between particle size and sinking velocity. Second, because of the labor and expense involved in obtaining these data, they were obtained at only a few depths, and their analysis required constructing a new type of box-like model, which we refer to as a “two-layer” model, that we then analyzed using likelihood techniques. Likelihood techniques were developed in the 1930s by statisticians, and form the computational core of both Bayesian and non-Bayesian statistics. Their use has recently become very popular in ecology, but they are relatively unknown in geochemistry.Our model was formulated by assuming steady state and first-order reaction kinetics for thorium adsorption and desorption, and for particle aggregation, disaggregation, and remineralization. We adopted a cutoff settling velocity (49m/d) from Armstrong et al. (2009) to separate particles into fast- and slow-sinking classes. A unique set of parameters with no dependence on prior values was obtained. Adsorption rate constants for both slow- and fast-sinking particles are slightly higher in the upper layer than in the lower layer. Slow-sinking particles have higher adsorption rate constants than fast-sinking particles. Desorption rate constants are higher in the lower layer (slow-sinking particles: 13.17±1.61, fast-sinking particles: 13.96±0.48) than in the upper layer (slow-sinking particles: 7.87±0.60y−1, fast-sinking particles: 1.81±0.44y−1). Aggregation rate constants were higher, 1.88±0.04, in the upper layer and just 0.07±0.01y−1 in the lower layer. Disaggregation rate constants were just 0.30±0.10y−1 in the upper layer and higher, 3.01±0.35y−1, in the lower layer. For rapid-sinking particles, adsorption of Th from solution and aggregation of slow-sinking particles are the main source, and particle sinking is the main sink. For slow-sinking particles, adsorption is also the main source, but radioactive decay (for 234Th) and desorption are the main sinks. Sinking and aggregation are small compared to adsorption and decay, and the contribution of disaggregation is negligible.

Variability in the abundance and species composition of diatoms in sinking particles in the northern South China Sea: Results from time-series moored sediment traps

Seasonal variability of diatom abundance and species composition in sinking particles in the northern South China Sea was studied using continuous time-series sediment trap moorings during a one-year period, 2009–2010. A clear seasonal pattern of diatom abundance was observed, with higher fluxes in winter than in summer. This could be related to positive response of diatom growth to increased nutrient supply from the subsurface waters as a result of enhanced vertical mixing in winter. Mesoscale eddy activities and strong Asian dust input could also stimulate the diatom growth and increase the diatom export flux to the deep South China Sea. The strong Asian dust input during spring 2010 probably accelerated the aggregation and downward flux of diatoms in the northern South China Sea. No distinctive seasonal pattern in diatom species composition was recorded, but there was a close correlation of diatom abundances with the cell size and the degree of silification. The sinking velocity of the diatom related particles from the sea surface to the deep South China Sea has been estimated to be ~30–50md−1 by comparing the surface chlorophyll a with the diatom abundance and species in sediment traps at 1003m and 3226m depths. Strong dissolution of small-sized and low-silicified diatom species and lateral advection of particles were also indicated for the deep northern South China Sea. The present results are crucial as a basis for future understanding of fossil assemblage changes in connection with palaeoclimatic and palaeooceanographic research.

Effects of sinking velocities and microbial respiration rates on the attenuation of particulate carbon fluxes through the mesopelagic zone

AbstractThe attenuation of sinking particle fluxes through the mesopelagic zone is an important process that controls the sequestration of carbon and the distribution of other elements throughout the oceans. Case studies at two contrasting sites, the oligotrophic regime of the Bermuda Atlantic Time‐series Study (BATS) and the mesotrophic waters of the west Antarctic Peninsula (WAP) sector of the Southern Ocean, revealed large differences in the rates of particle‐attached microbial respiration and the average sinking velocities of marine particles, two parameters that affect the transfer efficiency of particulate matter from the base of the euphotic zone into the deep ocean. Rapid average sinking velocities of 270 ± 150 m d−1 were observed along the WAP, whereas the average velocity was 49 ± 25 m d−1 at the BATS site. Respiration rates of particle‐attached microbes were measured using novel RESPIRE (REspiration of Sinking Particles In the subsuRface ocEan) sediment traps that first intercepts sinking particles then incubates them in situ. RESPIRE experiments yielded flux‐normalized respiration rates of 0.4 ± 0.1 day−1 at BATS when excluding an outlier of 1.52 day−1, while these rates were undetectable along the WAP (0.01 ± 0.02 day−1). At BATS, flux‐normalized respiration rates decreased exponentially with respect to depth below the euphotic zone with a 75% reduction between the 150 and 500 m depths. These findings provide quantitative and mechanistic insights into the processes that control the transfer efficiency of particle flux through the mesopelagic and its variability throughout the global oceans.